import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;
import java.util.Queue;
import java.util.concurrent.LinkedBlockingDeque;

class Solution {
    class Solution {
        public List<List<Integer>> levelOrder(TreeNode root) {

            List<List<Integer>> list = new ArrayList<>();
            if(root==null) return list;
            Queue<TreeNode> queue = new LinkedList<>();
            queue.offer(root);
            while(!queue.isEmpty()) {
                int size=queue.size();
                List<Integer> ret = new ArrayList<>();
                while(size!=0) {
                    TreeNode cur = queue.poll();
                    ret.add(cur.val);
                    size--;
                    if(cur.left!=null) {
                        queue.offer(cur.left);
                    }
                    if(cur.right!=null) {
                        queue.offer(cur.right);
                    }
                }
                list.add(ret);
            }
            return list;
        }
    }
}